JNITED STATES DEPARTMENT OF AGRICULTURE 

I BULLETIN No. 577 v. 




Contribution from the Bureau of Plant Industry 
WM. A. TAYLOR, Chief 




J^^^Ji. 



Washington, D. G. 



PROFESSIONAL PAPER 



September 14, 1917 



EXPERIMENTS IN THE CONTROL OF POTATO LEAK.^ 

By LoN A. Hawkins, Plant Physiologist, Plant Physiological and Fermentation 
Investigations, Bureau of Plant Industry. 



CONTENTS. 



The causal organisms. . . 
Methods of investigation. 



1 Results of the experiments. 

2 I Conclusions 



THE CAUSAL ORGANISMS. 

In a paper ^ on a disease of potatoes commonly known as leak, the 
disease was described and the literature on the subject reviewed. It 
was shown that the tuber rot typical of this disease could be caused 
by Rhizopus nigricans Ehrenb., as Orton ^ found, but that the causal 
organism was more frequently Pythium deharyanum Hesse. The 
fungus obtains entrance through breaks in the skin of the potatoes, 
and infection under field conditions usually takes place by some in- 
fected soil getting into wounds made in harvesting. Preliminary 
experiments in 1915 indicated that the disease could be controlled 
by carefully sorting out aU wounded tubers. As the disease is of 
considerable economic importance in the delta region of San Joaquin 
County, Cal., it was considered advisable to continue these experi- 
ments. This bulletin deals with further experiments in the control 
of the disease, together with some additional work on the causal 
organism and its occurrence in the delta soils. 

1 The work described in this bulletin was carried out as a part of the potato-disease project of the OiQce 
of Cotton and Truck Disease Investigations. The writer wishes to acknowledge his indebtedness to Mr. 
Carson C. Cook, of Stockton, Cal., for his hearty cooperation in this work. 

2 Hawkins, L. A. The disease of potatoes known as "leak." In Jour. Agr. Research, v. 6, no. 17, pp. 
627-640, 1 fig., pi. 90. 1916. 

« Orton, W. A. Potato diseases in San Joaquin county, California. U. S. Dept. Agr., Bur. Plant Indus. 
Circ. 23, 14 p. 1909. 

Decay of potatoes due to Rhizopus nigricans. (Abstract.) In Science, n. s., v. 29, no. 753, 

p. 916. 1909. 

1903"— Bull. 677—17 



Id ^ 



2 BULLETIN 577, V. S. DEPARTMENT OF AGRICULTUEE. r Y ^^ 

The fungus was isolated from the rotten tubers and inoculatioiis 
made, as in the previous year. The results were similar to those 
obtained in 1915. Forty isolations of Pythium deharyanum were 
made in the 45 attempts. The pathogenicity of the organism was 
demonstrated in many instances by inoculating it into sound tubers 
and reisolating it after the potato had rotted. Inoculations were 
made by inserting small quantities of soil from various potato fields 
into tubers. About 50 per cent of the inoculations produced the 
disease from every sample of soil obtained from land which had been 
reclaimed and farmed for some years and had not been burned over 
recently. 

METHODS OF INVESTIGATION. 

It is considered among the growers that potatoes would not leak 
if grown on new land (that is, land recently reclaimed) or on burned 
land (that is, land on which the peat soil had caught fire and burned 
more or less deeply). This theory was tested. A- number of inocula- 
tions were made from samples of sod from an Vea that had been 
burned over. In no case was there any infection. Samples of soil 
from two areas of new land, one of which had never been farmed, 
while the other had been planted to potatoes the previous year, were 
tested. Two potatoes out of eight inoculated with sod from the land 
most recently reclaimed rotted, and Rhizopus nigricans was isolated 
from the tubers. AH other inoculations gave negative results. 
Potatoes from such lands are not immune when inoculated with 
Pythium debaryanum. The apparent immunity of potatoes grown 
on new land or burned land seems to be due to the absence of the 
organism from the soil or to its presence only in hmited areas. 

The general plan of the experiments to demonstrate a method for 
the control of the disease was to sort the potatoes in the field and 
store the injured tubers imder conditions as nearly approaching 
those of a commercial warehouse as possible. The sound tubers 
were to be shipped in the usual way. It was planned to examine 
the cars of soimd potatoes after they reached the market or after 
they had been shipped and sufficient time had elapsed for the in- 
cubation of the fungus. 

The farm selected for the experiments was one on which a number 
of crops of potatoes had been grown. Several cars of the 1916 crop 
had been shipped and considerable damage from leak had been re- 
ported. In the experiments here described the potatoes were 
harvested and sacked in the field in the usual way. They were then 
hauled to the levee, sorted, resacked, and the sound potatoes shipped. 
The tubers that had been wounded by the removal of branches 
(knobs) and by the digging fork were sorted out, and each lot was 
sacked and stored separately. The wounded tubers remained in a 
weU-ventilated warehouse on the levee imtil sufficient time had 

OCT *6 1917 



EXPERIMENTS IN THE CONTROL OF POTATO LEAK. 3 

elapsed for the incubation of the fungus in any inoculated tubers. 
They were then resorted and the number of rotten potatoes in each 
sack noted. The wounded tubers which had not rotted were sacked 
and sold. 

RESULTS OF THE EXPERIMENTS. 

Tlie first car of potatoes included in these experiments was har- 
vested and sorted on September 4 and 5. For the car of sound 
potatoes (270 sacks), 349 sacks of field-i*un potatoes were required. 
Of the 79 sacks sorted out, 43 sacks were of tubers which had been 
wounded by the removal of knobs, and 25 sacks had been injured 
with the digging forks. The other 11 sacks represent shrinkage 
due to the drying of the potatoes and to the stretching of the sacks 
used in resacking. 

This car of sound potatoes was shipped to Oklahoma. It was 
impossible for tiie wTiter to examine the potatoes after the car left 
Stockton, Cal. No leak had been reported by the consignee on 
October 1, so it is reasonable to suppose that the potatoes arrived in 
good condition. 

The injured tubers sorted out were all stored in the warehouse 
until September 13, when part of them were sorted and the unin- 
fected potatoes sold. The remaining tubers were sorted on Septem- 
ber 18, 14 days after the first of the lot were harvested. 

The second and third cars of potatoes were harvested and sorted 
from September 8 to 12, inclusive. For the second car (270 sacks), 
326 sacks of field-run potatoes were required. Of the 56 sacks 
sorted out, 20 sacks were of tubers that had been injured by the 
digging forks, and 31 sacks were those from which knobs had been 
removed. The other 5 sacks represent shrinkage. The car of sound 
potatoes was shipped to Barstow, Cal., and was examined there by 
the writer six days after the potatoes had been harvested. No leaky 
tubers were found in any of the 20 sacks examined. Some leak was 
reported from Phoenix, Ariz., where the car was finally sent. 

The third car of potatoes in these experiments was sorted imme- 
diately after the second. It required 325 sacks of field-run potatoes 
for the car of sound potatoes. Of the 55 sacks sorted out, 20 sacks 
were of tubers that had been injured by the removal of knobs, and 
21 sacks were of those injured by the digging forks. The other 4 
sacks represent slu^inkage. The car of sound potatoes was shipped 
to Bakersfield, Cal., where it was examined six days after most of 
the potatoes in it had been harvested. No leaky tubers were found. 
The car was diverted from Bakersfield to Los Angeles, where it was 
reported to be leaking. It was then shipped to El Centro, Cal., 
where the potatoes were examined by the writer. The merchant to 
whom the potatoes were sold said that they arrived in fair condition 



4 BULLETIN" 577, U. S. DEPARTMENT OF AGRICULTURE. 

and that about 1 sack in 20 contained diseased tubers. A number 
of sacks were examined; most of them were free from disease; in 
only one sack was there more than one diseased tuber, and all the 
diseased tubers found had been wounded with the digging fork. 
The amount of leak in the car was considered too small to warrant 
sorting. 

The injured potatoes from the second and third cars were stored 
in the warehouse until September 18. They were then sorted and 
the uninfected potatoes sold. The data obtained from sorting the 
1,000 sacks of potatoes as they came from the field are summarized 
in Table I, which includes the data ah-eady given, together with that 
obtained in sorting the wounded tubers. 

Table I. — Experiments with 1,000 sachs of potatoes harvested in the us^ial way in Cal- 
ifornia, showing the results of sorting for the control of potato leak. 



Car of 


Field- 
run 
pota- 
toes 
re- 
quir- 
ed. 


Shrink- 
age in 
sorting 


Tubers injured — 


Rotten tubers (percentage based on estimate of 200 
tubers to each sack) in three lots of those injured— 


In- 
jured 
tubers 
mar- 
ket- 


sorted 

tubers 

(270 


By 

remov- 
al of 

knobs. 

Sacks. 
43 
31 
30 


By 

fork 

wounds. 


In 

field- 
run 
lots. 


By removal of knobs. 


By fork wounds. 


sacks). 


Maxi- 
mum. 


Mini- 
mum. 


Average. 


Maxi- 
mum. 


Mini- 
mum. 


Average. 


when 
sorted. 


No. 1.... 

No. 2.... 
No. 3.... 


Sacks. 
349 
326 
325 


Sacks. 
11 
5 
4 


Sacks. 
25 
20 
21 


Per 
cent. 
19.4-1- 
15.6-1- 
15.6+ 


Num- 
ber. 
23 
40 
35 


Num- 
ber. 
2 
8 
10 


Num- 
ber. 
12.5 
17 
19 


Per 

cent. 

6.25 

8.5 

9.5 


Num- 
ber. 
61 
75 
68 


Num- 
ber. 
14 
21 
28 


Num- 
ber. 
40 
48 
49 


Per 
cent. 
20 

24 
24.5 


Sacks. 
56.5 
40 
40 



Table I shows that something more than 19.4 per cent of the pota- 
toes in the first lot sorted were wounded in harvesting, either by 
the removal of branches (knobs) or by the digging fork. The 
percentage of injured tubers in the other two lots, 15.6, is not quite 
so great. In all, 170 sacks of potatoes out of 1,000 were injured. 
That such a percentage of the potatoes should be injured in harvest- 
ing indicates exceedingly poor methods. Nor does this 170 sacks 
represent all the injured potatoes, as the tubers injured by fork 
wounds were supposed to have been sorted out in the field. Much 
of this damage might be eliminated with proper care, but as long as 
the potatoes are harvested with forks it is probable that there will 
be a high percentage of injured tubers. The digging of potatoes 
with machinery is practiced very httle in this region, though potato 
harvesters adapted to this type of soil and these conditions are said 
to be available. 

Of the 170 sacks of injured potatoes, 104 sacks (10.4 per cent of 
the total of 1 ,000 sacks) were injured by the removal of knobs. Break- 
ing off the knobs is a common practice in this region and is recom- 
mended by the potato buyers, as it results, of course, in a smoother 
potato. The tubers injured in this way are considered to be no more 



EXPERIMENTS IN THE CONTROL OF POTATO LEAK. 5 

subject to leak than sound ones. That this view is erroneous is evi- 
dent from the results obtained in these experiments. Of the first 
lot, 6.25 per cent of the potatoes injured in this way became infected, 
wliile in the second and third lots the percentage was somewhat 
higher, being 8.5 and 9.5, respectively. The practices of removing 
these branches and of rubbing the raw surface in the soil, as is 
sometimes done, or sacking them, so that soil gets into the fresh 
wound, are harmful. If the broken surface is exposed to the air 
until the wound has had time to cork over, there is much less danger 
of infection. 

Not so many potatoes were wounded with the digging forks as by 
the removal of knobs, but the percentage of rotten ones among 
the fork-wounded tubers was much higher. Out of 1,000 sacks har- 
vested, 66 sacks were wounded with the digging forks. Of these, 
from 20 to 24.5 per cent rotted. The fact that the deep wounds 
made with the digging fork do not dry so readily and are more lia- 
ble to be filled with infected soil than wounds made by the removal 
of knobs seems to account for this higher percentage of infection. 

CONCLUSIONS. 

It is evident from these experiments that the sorting out of all 
wounded tubers would practically insure a shipment from damage 
by tliis disease. It is probable that it would be impracticable in 
commercial work to sort carefully enough to eUminate aU the 
wounded tubers. A few wounded potatoes slipped tlirough in these 
experiments in spite of the careful sorting. It is not difficult, how- 
ever, to remove 96 to 98 per cent of them. Such sorting, according 
to the percentages of rot occurring in the wounded tubers in these 
experiments, would result in an average of about one rotten potato 
in 10 to 25 sacks. This would be nearly complete control. 

It has therefore been demonstrated that the leak may be con- 
trolled by the elimination of aU wounded tubers from the shipment. 
The best and most economical means of accomplishing this end would 
be to avoid wounding the tubers in harvesting them. Wounded 
tubers should be sorted out and shipped separately or, better, stored 
for a week or more to aUow the fungus to incubate, and then sorted 
and the uninfected potatoes sold. 



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